Traditional or 5-Point plate?

I'm a lower-level 3-day eventer (BN/N currently, but planning on advancing up the levels). Just got a new horse in December (appendix mare, 15.3, cob-size) and her saddle seems to keep slipping backwards. She also tends to get high-headed when jumping. I've concluded that I need to buy a plate with a running martingale attachment. The question is: Should I go traditional, or get one of the more effective and fashionable 5-point plates? I've been looking on ebay and I can get one pretty cheap on there (they usually cost around $200-$300). Any personal opinions or straight-up facts are welcome.